Paper machine clothing is the term for industrial fabrics used on paper machines in the forming, pressing and drying sections. They are generally fabricated with either polyester or polyamide multifilaments and/or monofilaments woven on conventional, large textile looms. These fabrics have been fabricated by conventional weaving techniques. The materials and processes, although an industry standard, have some inherent limitations described below.
The primary function of all paper machine clothing (PMC) is removal of water from the paper sheet. As both the manufacturer of paper machine builder and papermaker work to increase the speed of the papermaking process and improve paper quality, new barriers have been identified for PMC fabrics that demand innovation in materials and fabric design. Furthermore, the PMC manufacturer is also looking for more efficient production of PMC fabrics and enhancing key quality characteristics of the same.
Today, paper making machines are attaining such rapid speeds that the thickness of the fabric structure is beginning to limit the rate of water removal, especially in the forming section. Insufficient dewatering results in low sheet strength. Sheet strength is critical for transferring and maintaining sheet properties through the next, more aggressive stages of sheet dewatering. One possible solution is to lengthen the forming section of the machine, but this is rather expensive and therefore of limited viability. The other approach is for the PMC manufacturer to produce thinner fabrics, but in a weaving process the smallest possible dimensions are the combined diameters of the filaments used in the warp and shute directions. Criteria such as dimensional stability, fabric strength and fabric life result in a practical limit to the fineness of the filament diameter and thus the overall thickness of the fabric. In many PMC positions, a tradeoff of these properties is not feasible or practical, and in fact higher machine speeds actually require further enhancement of these properties.
PMC fabrics are also porous media that must effectively achieve fluid flow, that is, either water flow in forming and pressing or air flow in drying. The porosity of the fabrics can greatly affect sheet properties important in the forming and pressing sections of the paper machine. Channels for transport are formed by the open spaces or interstices, between the warp and shute yarns. Channels also exist between the filaments at the crossover points. The weaving process limits the geometry of the pores because the yarn filaments are orthogonal.
The surface topography of PMC fabrics contributes to the quality of the paper product. Efforts have been made to create a smoother contact surface with the paper sheet. However, surface smoothness of PMC woven fabrics is limited by the topography resulting from the weave pattern and the filament physical properties. In a woven fabric (or knitted fabric), smoothness is inherently limited by the knuckles formed at the cross-over point of intersecting yarns.
PMC fabrics require constant cleaning because of build-up materials from the paper furnish. Two mechanisms of fabric soiling have been identified. Mechanical bonding occurs when fine particles from the paper furnish are entrapped in the spaces existing between filaments in the fabric. This mechanical bonding is enhanced by the fine interstices created at the orthogonal cross over points in a woven fabric. Chemical bonding describes the adherence of fine particles that comprise the furnish to the fabric due to the existence of chemical affinities. This problem has been studied over many years of effort and results indicate that mechanical bonding is more important than chemical bonding overall. Decreasing permeability from particle build-up decreases the useful life of a fabric. High pressure showers have been employed to wash the fabrics, but the harsh abrasive environment these showers present also decreases the useful life of PMC fabrics.
PMC manufacturing technology could be improved by speeding the weaving process. In weaving, a warp is threaded through a heddle, and the weave pattern is created by raising and lowering the heddle position for each filament in the warp direction before the shute pick. This is a slow process due to its many steps. A practical production rate for typical forming, pressing or dryer loom is limited to 100 picks/minute.
A variety of forming fabrics based largely upon polyester monofilaments have been developed in the past few decades. The most advanced of these developments is a two-layer monofilament fabric in which the two fabric layers are held together via a binder monofilament. Commercially, this fabric is sold under the name Triotex.RTM. by Albany International Corp., Albany, N.Y. The binder monofilament is the only monofilament in the Triotex.RTM. structure that holds the two fabric layers together. The top fabric layer is usually a plain weave structure, which is designed for optimal paper sheet formation. The bottom fabric layer is designed for wear and typically has long floats in which the shute monofilament travels under three or more warp monofilaments. These long floats are used as an abrasive wear surface, which wears away before wear can occur to the warp monofilaments. The binder monofilament is a shute monofilament that mechanically holds the top and bottom fabric layers together by traveling over a warp monofilament in the top fabric layer and under a warp monofilament in the bottom fabric layer. Under running conditions, the bottom and top fabric layers move relative to each other. This relative movement leads to fatigue and wear of the binder monofilament due to repeated deflection back and forth within the structure. Eventually, the binder monofilament will fail and allow the top and bottom fabrics to separate from each other. This separation leads to product failure.
PMC press fabrics are constructed from woven base fabrics of monofilaments and multifilaments. A carded web of staple filaments is needled onto the base fabric, forming a construction capable of transporting water away from the forming sheet of paper. Needling can damage the monofilaments in the base fabric, weakening the fabric. Press fabrics are also prone to shedding, the release of the batt fibers from the felt. Shedding results in a contaminated paper sheet and shortens the useful life of the press fabric. Paper sheet rewetting is often a problem in press fabrics. Fluid removed from the sheet in the press nip can return to the sheet immediately after exiting the nip, reducing the overall efficiency of the pressing operation.
U.S. Pat. No. 4,740,409 discloses a nonwoven fabric having knuckle-free planar surfaces comprised of parallel linear machine direction yarns residing in a single plane and interconnecting, cross-machine direction polymeric material also residing in the plain, the cross machine direction material entirely surrounding the machine direction yarns. An array of side by side sheath core yarns are fed to machine direction grooves of a pinned roll section where they are forced into the grooves by heat and pressure. The sheath core monofilament cross section area is greater than the area of the machine direction groove so that excess sheath material is forced into cross direction grooves to form the cross directional interconnecting structure.
U.S. Pat. No. 5,077,116 discloses a forming fabric having a non-woven surface coating. The forming fabrics have a transverse nonwoven sheet contact layer adhered to the base fabric layer. The fluid flow passageways between adjacent structured members in the nonwoven sheet contact layer are smaller than the fluid flow passageways in the adjacent base fabric layer and are in fluid communication with the nonwoven sheet contact surface or the nonwoven surface adjacent the base fabric, or both. The nonwoven sheet contact layer may be comprised of bicomponent fibers having a polyester core and low melting temperature copolyester sheath. It is disclosed that these fibers could be adhered to each other and to the base fabric by fusion bonding means.
U.S. Pat. No. 5,366,797 discloses a bonded yarn bundle comprising at least one twisted multifilament yarn composed of a first synthetic polymer, whose individual filaments have become bonded together over essentially the entire thread cross-section by the melting of a second thermoplastic synthetic polymer whose melting point is at least 10.degree. C. below the melting or decomposition point of the first synthetic polymer.
The yarn bundles comprised of a yarn of a first synthetic polymer is a meltable or nonmeltable polymer which provides a high strength characteristic. The yarn of a second synthetic polymer is a meltable material whose melting point is lower than the melting point of the first material.
GB 2 097 435 discloses a papermaker's fabric using yarns woven from high melting point monofilament or multifilament warp yarns and similar top and bottom weft yarns. Stiffer weft yarns in the center plane of the fabric are lower melting point synthetic yarns. The fabric is heated to a temperature to cause the low melt temperature stuffer yarns to melt and flow in a way that they fill voids in the weave pattern, reducing permeability.
U.S. Pat. No. 4,731,281 discloses a papermaker's fabric, woven from uniformly precoated, totally encapsulated monofilament yarns. The yarns are coated prior to the weaving of the papermaker's fabric in order to impart anti-sticking characteristics to the papermaker's fabric. The coatings may be such that thickness of the machine direction yarns is different than the thickness of the cross-machine direction yarns.